The interactions or behaviors of the spin states of electrons in the presence of magnetized materials have provided the basis for development of many solid-state magnetoelectronic and spintronic devices. For example, some multi-layer storage devices containing magnetized ferromagnetic materials use giant magnetoresistance (GMR), where a relatively small change in magnetism causes a large change in resistance. Some other spintronic devices similarly use large tunnel magnetoresistance (“TMR”), which can be observed in structures including an insulating tunnel barrier sandwiched between two magnetized ferromagnetic layers of similar or different materials.
A class of solid-state devices that is currently better known than spintronic or magnetoelectronic devices is semiconductor light sources. Semiconductor light sources include such devices as light-emitting diodes (LEDs) and semiconductor lasers, e.g., vertical cavity surface emitting lasers (VCSELs) and edge-emitting lasers. The technology required for fabrication and operation of these semiconductor light source is relatively mature and generally well known in the art. However, development of solid-state sources of polarized radiation, i.e., lasers and LEDs that produce polarized light, is an urgent problem for optical communication because conventional solid-state light sources generally produce light with a low degree of polarization.
FIG. 1A illustrates a schematic model of a conventional light-emitting diode (LED) 100. LED 100 includes three adjacent semiconductor layers 110, 120, and 130, an underlying substrate 140, and electrical contacts 150 and 160 respectively above semiconductor layer 110 and below the substrate 140. In a typical configuration, the top and bottom semiconductor layers 110 and 130 are both relatively heavily doped with impurities of the opposite conductivity types. For example, in the configuration illustrated in FIG. 1A, semiconductor layer 110 is an n+ layer, and semiconductor layer 130 is a p+ layer.
The middle semiconductor layer 120, which is sometimes referred to as an activation or quantum well layer, may be either positively or negatively doped, but semiconductor layer 120 generally has a band gap that is narrower than the band gaps in the adjacent semiconductor layers 110 and 130. As a result, semiconductor layer 120 is associated with a quantum well, which can attract or hold low energy conduction electrons and holes for recombination.
FIG. 1B illustrates a typical energy band diagram of electron states in LED 100 at equilibrium. FIG. 1B more specifically shows the Fermi level EF, the bottom conduction band energy level EC, and the top valence band energy level EV in each semiconductor layer 110, 120, and 130. FIG. 1B also shows the energy band gaps Eg1, Eg2 and Eg3 for respective semiconductor layers 110, 120, 130, where Egi=ECi−EVi when index i is 1, 2, and 3 respectively for layers 110, 120, and 130. As mentioned above, the energy band gap Eg2 in layer 120 is less than energy band gaps Eg1 and Eg3 of respective semiconductor layers 110 and 130, i.e., the energy band gaps satisfy the relation Eg2<Eg1,Eg3.
FIG. 1C illustrates the injection of minority charge carriers (in this case electrons) into semiconductor layer 120 when a forward bias is applied between contacts 150 and 160. Recombination of electrons 122 and holes 124 in semiconductor layer 120 of LED 100 generates radiation (i.e., photons 135). In the configuration of FIG. 1A, LED 100 is an edge-emitting device, and photons escape from a cut edge of LED 100. The angular frequency ω and polarization of each photon emitted when an electron 122 recombines with a hole 124 depends on the energy difference of the quantum states of electron 122 and hole 124 and the spin/angular momentum states of electron 122 and hole 124. Generally, electrons 122 and holes 124 are only slightly spin-polarized (due to a weak spin-orbital coupling) in LED 100. Consequently, the radiation has a very low degree of polarization.
Efficient solid-state devices and methods for producing polarized radiation are thus sought.